Field of the Invention
This invention relates generally to ceramic matrix composite (CMC) structures, and more particularly to CMC structures for turbine airfoils.
Description of the Related Art
CMC structures find use in applications that require components capable of withstanding high loads and high temperatures. One such application is the gas turbine, which includes numerous components—including the turbine's vanes or blades (generally, “airfoils”)—that are subjected to adverse conditions.
For optimum engine performance, it is generally desirable to have the trailing edges of the turbine airfoils be extremely thin. Previous developments of ceramic composite airfoils for turbine engines have relied mostly on the use of multi-ply 2-D layups of reinforcing fibers. However, such 2-D lay-up structures face several limitations. One is a low through-thickness strength and susceptibility to delamination, especially in the vicinity of a sharp trailing edge and in regions near junctions of the airfoil skin and internal walls.
Another limitation concerns the common requirement for rows of closely spaced cooling holes near the airfoil's leading and trailing edges. The conventional approach of forming the holes by laser drilling after processing of the composite destroys reinforcing fibers, causes damage in the surrounding matrix, and results in severe strength loss in the composite.
Previous attempts to overcome these problems are limited. In the NASA UEET program, a method was developed to provide interlocking of fiber tows that form the sharp trailing edge by use of a Y-fabric, formed by merging two fabrics together during weaving. Although this adds integrity to the tip of the trailing edge, the Y-fabric is only one layer of a 2-D layup and thus does not solve the problem of susceptibility to delamination elsewhere on the airfoil. Moreover, the weaving process used to form the Y-fabric, with warp fibers oriented normal to the trailing edge, is not capable of creating the topology of a closed-wall figure with unbroken interlocked fiber paths around the circumference, as would be required for a continuously reinforced airfoil.
Other textile methods such as braiding and knitting are capable of producing closed airfoil shapes. However, knitting is limited to creating low volume fractions of fibers and would be especially limited with high performance SiC fibers. Braiding is capable of producing high volume fractions of fibers and has the appealing attribute that relatively sharp edges can be formed by passing fiber tows over the edge at a low angle in order to avoid bending the fibers to a small radius of curvature. However, the braiding process is not capable of deploying fibers primarily in two orthogonal directions (radial and circumferential) as needed for optimal design of blades and vanes.